In mobile telephone systems, in which the mobile units are controlled by active cards assigned to the subscribers, the personalization of the respective card constitutes an essential procedure which is safeguarded by rigorous security measures and which includes activating the card and loading it with IMSI and Ki, and preferably also a so-called PUK code (Personal Unblocking Key). It is essential that this loading be effected in a safe manner to prevent unauthorised people from having access thereto.
In view hereof, the personalization procedure is carried out in a central personalization place or a customer service place adjacent the central computer, where it is possible to meet the high demands on security.